1. Field of the Invention
The invention relates to projection objectives of microlithographic projection exposure apparatus. Such apparatus are used for the production of large-scale integrated circuits and other microstructured components. The invention relates in particular to projection objectives comprising a manipulator for reducing rotationally asymmetric image errors.
2. Description of Related Art
Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for a aligning the mask, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of an rectangular slit or a narrow ring segment.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection lens, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning means. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193. Also programmable LCD arrays may be used as active masks, as is described in U.S. Pat. No. 5,229,872. For the sake of simplicity, the rest of this text may specifically relate to apparatus comprising a mask and a mask stage; however, the general principles discussed in such apparatus should be seen in the broader context of the patterning means as hereabove set forth.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically generate structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
The size of the structures which can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the (deep) ultraviolet spectral range.
Another way of increasing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an immersion interspace, which remains between a last lens on the image side of the projection objective and the photoresist or another photosensitive layer to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.4 or even higher.
The correction of image errors (i.e. aberrations) is becoming more and more important for projection objectives with particularly high resolution. Many ways in which image errors can be corrected in projection objectives are known in the prior art.
The correction of rotationally symmetric image errors is comparatively straightforward. An image error is referred to as being rotationally symmetric if the wavefront deformation in the exit pupil is rotationally symmetric. The term wavefront deformation refers to the deviation of a wave from the ideal aberration-free wave. Rotationally symmetric image errors can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those image errors which are not rotationally symmetric is more difficult. Such image errors occur, for example, because lenses and other optical elements heat up rotationally asymmetrically. One image error of this type is astigmatism, which may also be encountered for the field point lying on the optical axis. Causes of rotationally asymmetric image errors may, for example, be a rotationally asymmetric, in particular slit-shaped, illumination of the mask, as is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illumination field causes a non-uniform heating of the optical elements, and this induces image errors which often have a twofold symmetry.
However, image errors with other symmetries, for example threefold or fivefold, or image errors characterized by completely asymmetric wavefront deformations are frequently observed in projection objectives. Completely asymmetric image errors are often caused by material defects which are statistically distributed over the optical elements contained in the projection objective.
In order to correct rotationally asymmetric image errors, U.S. Pat. No. 6,338,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. Since the two optical surfaces of the deformable lens are always deformed simultaneously, the overall corrective effect is obtained as a superposition of the individual effects caused by the two deformed optical surfaces. This is disadvantageous because it is thereby very difficult to correct a particular wavefront deformation, which is determined by measurements or simulation, and has been generated by the other elements of the projection objective. The individual effects of the two deformed surfaces furthermore partially compensate for each other, so that the lens has to be deformed quite strongly in order to obtain a sufficient corrective effect.
US Pat. Appl. No. 2001/0008440 A1 discloses a manipulator suitable to correct image errors for a projection objective, in which two membranes or thin plane-parallel plates enclose a cavity. The membranes or plates can be deformed by varying the pressure of a fluid (a gas or a liquid) contained in the cavity. A rotationally asymmetric deformation can be achieved by rotationally asymmetric framing of the membranes or plates. A disadvantage with this known manipulator, however, is that only the fluid pressure is available as a variable parameter. This implies, for example, that the symmetry of the deformation is once and for all fixed by the framing of the membranes or plates and thus cannot be changed.
WO 2006/053751 A2 discloses various adjustable manipulators for reducing a field curvature. In some embodiment optical elements such as lenses or membranes are deformed by changing the pressure of liquids adjacent the optical elements.
U.S. Pat. No. 5,665,275 discloses a prism which has a variable prism angle. A variation of the prism angle is achieved using two plane-parallel plates which can be mutually deflected via a deformable connecting ring. The interspace defined by the plates and the connecting ring is filled with an organic liquid. The connecting ring is deformed by an actuator engaging on it. This device is particularly suited for an anti-vibration optical system in a photographic system.
U.S. Pat. No. 5,684,637 discloses a spectacle lens having a cavity which is filled with a liquid. For the correction of rotationally asymmetric image errors, for example astigmatism, deformable membranes are provided that have a non-circular circumference. Since always a plurality of optical surfaces are simultaneously deformed by changing the pressure of the liquid, it is difficult to widely correct a certain (measured) wavefront deformation without introducing a plurality of other deformations.
There are many other liquid lenses in the prior art having deformable lens surfaces for changing the focal length. With these known liquid lenses the deformation is always rotationally symmetric, and hence they are not suitable for correcting rotationally asymmetric image errors. Examples of such variable focal length lenses can be found in Japanese Pat. Appls. JP 811 4703 A, JP 2002 131 513 A and JP 2001 013 306 A, in EP 0 291 596 B1 and in U.S. Pat. No. 4,289,379.